1. Field of the Invention
The present invention is directed generally to methods and apparatus used for growing films, and in particular, methods and apparatus using chemical vapor deposition techniques in a rapid thermal processing system for growing films.
2. Description of the Background
In the fabrication of semi-conductor devices, the deposition of a film on a surface of the wafer is a common and necessary step. The film is typically a semi-conductor, a conductor, or a dielectric. It is well known in the prior art that film deposition occurs more readily on a hot surface than on a cold surface. As a result, it is necessary to heat the surface of the wafer to induce film deposition. Wafers are typically heated and processed either by conventional batch furnace processing or by rapid thermal processing ("RTP").
RTP is an alternative to conventional batch furnace processing and is characterized by short processing times and rapid thermal rise and fall rates. An RTP process step typically lasts between several seconds and 15 minutes, with thermal rise rates typically between 100 and 500.degree. C. per second, and reaching temperatures of 1200.degree. C.
RTP has applications in the fabrication of very large scale integrated ("VLSI") circuits and ultra large scale integrated ("ULSI") circuits. In particular, RTP is used in the fabrication steps of thermal oxidation, thermal nitridation, dopant diffusion, thermal annealing, refractory metal silicide formation, and chemical vapor deposition ("CVD"). CVD may be used to form semi-conductive, conductive, and dielectric films. The design of RTP reactors is well known in the prior art, as disclosed, for example, in U.S. Pat. No. 5,446,825, issued to Moslehi et al., and U.S. Pat. No. 5,444,217, issued to Moore et al. An RTP reactor typically comprises a reaction chamber, a wafer handling system, a gas dispersion apparatus, a heat source, a temperature control system, and a gas control system.
The heat source is often high power lamps which drive chemical reactions in the reaction chamber and heat the wafer, thereby inducing film deposition on the surface of the wafer. The use of both single and multiple lamps is known, as disclosed in U.S. Pat. No. 5,444,217, issued to Moore et al. The gas dispersion apparatus introduces gases into the reaction chamber so that chemical reactions can take place and films can be deposited on the surface of the wafer. Many types of gas dispersion apparatus are known, and one or more may be located below the wafer, to the side of the wafer, or above the wafer.
CVD process steps are dependent on temperature. Because film is more readily deposited on hot surfaces than on cold surfaces, if the surface of a wafer is not a uniform temperature, a film will not be deposited uniformly. The temperature of a wafer, in turn, is dependent on the thickness of previously-deposited films. The absorption of energy, which is directly related to the temperature of the wafer, increases with the thickness of previously-deposited films.
It is well known, however, that the temperature of a wafer is not uniform. In particular, the temperature at the edge of a wafer tends to be significantly cooler than the temperature at the center, due to heat loss at the wafer edge. The lower temperature at the edge of the wafer results in slower deposition and a thinner film. That thinner film results in less absorption of energy, and a lower temperature than in the center of the wafer, perpetuating a feedback loop which exaggerates the non-uniformity of the wafer. The thickness of polysilicon, for example, can vary as much as 40% because of non-uniform temperatures along the surface of a wafer.
Uneven heating of wafers is undesirable and can cause slip dislocations, which are fractures in the crystal lattice that may lead to a device failure. Furthermore, an uneven surface can cause defects in subsequent process steps.
One type of gas dispersion apparatus used for CVD process steps is known as a "showerhead". Showerheads are located above the wafer, have a generally flat bottom surface with a plurality of gas ports therein, and provide for a generally uniform distribution of gas over the surface of the wafer. Showerheads are made from transparent materials which do not absorb much light, such as quartz.
Some solutions to the problems caused by non-uniform temperature of a wafer have been proposed. For example, U.S. Pat. No. 5,446,852, issued to Moslehi et al., discloses surrounding the wafer with a light absorbing graphite or silicon ring to reduce wafer edge cooling. This solution, however, is not satisfactory, particularly in CVD processes in an RTP chamber where there is sensitivity to temperature variations of less than 5.degree. C. Other solutions include the use of independently controlled light banks, using complex temperature sensors and complex computer software to constantly adjust the lamp banks in an attempt to provide uniform heating of a wafer. That approach is also deficient in extreme temperature sensitive applications, as well as being very costly. Thus, a need exists for a device which effectively provides uniform heating of the surface of a wafer.